Have you ever planted onions, only to find they didn’t grow the way you hoped? That’s exactly what happened to us when we unknowingly chose the wrong variety. Without realizing it, we picked short day onions, which are better suited for southern regions with shorter daylight hours. As a result, our onions struggled and refused to form proper bulbs in our northern garden.
After some research, we discovered the key difference between short day and long day onions and it changed everything! Now that we understand the importance of selecting the right variety, we’ve had much greater success with long day onions, which thrive in northern climates where summer days last 14-16 hours. These onions are ideal for regions with extended daylight, and they produce large bulbs when grown properly. We’ll share everything we’ve learned to help you grow your own long day onions from seed, even if you’re new to gardening. Let’s get started!
What are long day onions?
Long day onions need 14-16 hours of daylight, to form bulbs, making them perfect for northern climates with long summer days, usually best suited for hardiness zones 6 and colder. These onions often have a sharper flavour and store well for months after harvest. Popular varieties include Walla Walla, Yellow Sweet Spanish, and Blush.
Why grow onions from seed?
While you can grow onions from sets (small bulbs), starting from seeds allows you to:
Grow larger, healthier onions.
Access a wider variety of onions.
Save money, as seeds are more affordable than sets.
Step 1: Seed starting
Timing
Timing is important when starting onions from seed. Begin the process indoors during late winter, about 8-10 weeks before your last expected frost date. For example, if you’re in zone 5b, you should start your seeds around February. Starting early gives your onions plenty of time to grow strong before transplanting outdoors.
Seed starting medium
Choose the right growing medium to give your seeds the best start. You can use:
- High quality seed-starting mix or
- Homemade mix made from equal parts coco coir and worm castings.
If you’re using a homemade mix, make sure to moisten it slightly before filling your containers. This ensures the soil holds consistent moisture, which is essential for germination.
Containers
Onion seedlings don’t require deep containers because their roots are shallow. Use:
- Seed trays,
- Small pots, or
- Any shallow container with drainage holes.
Make sure the containers can drain excess water to prevent soggy soil and root problems.
Sowing the seeds
- Fill your containers with the seed starting mix.
- Make shallow holes about ½ inch deep in the soil.
- Sow 3-5 seeds per hole to ensure germination – later, you can thin them out as needed.
- Cover the seeds lightly with soil and gently press down to ensure good seed to soil contact.
Watering
After sowing, water the soil gently but thoroughly to moisten it evenly. Keep the soil consistently moist (but not waterlogged) during the germination process. You can cover the containers with a humidity dome or plastic wrap to retain moisture, but remove the cover once the seeds sprout.
Temperature & germination
Maintain a temperature of 65-75°F (18-24°C) for optimal germination. Under these conditions, onion seeds typically sprout within 7-14 days.
Step 2: Caring for onion seedlings
Provide adequate lighting
Once the seeds germinate, light becomes crucial for healthy growth. Place the containers in a bright location, we use grow lights for 12-16 hours daily. Without enough light, seedlings may become leggy and weak.
Strengthen the seedlings
To help your seedlings grow strong and upright, simulate the effect of wind by:
- Placing a small fan near them on a low setting, or
- Gently brushing the tops of the seedlings with your hand once or twice a day.
This strengthens their stems and prepares them for outdoor conditions.
Fertilizing
After the seedlings develop their first true leaves (the second set of leaves), begin feeding them with a diluted liquid fertilizer every 2-3 weeks. Organic options, like fish fertilizer with a bit of kelp, work well to encourage healthy growth.
Trimming the seedlings
When the seedlings reach 6 inches or taller, trim the leaves down to about 4-5 inches. Trimming prevents the tops from flopping over and encourages stronger root development. Repeat this trimming as needed while the seedlings grow indoors.
Step 3: Preparing for transplanting
Harden off the seedlings
Before planting your onions outdoors, they need to be gradually acclimated to outdoor conditions. This process, known as hardening off, should begin about 1-2 weeks before transplanting. Here’s how:
- Place the seedlings outside in a sheltered location for a few hours each day.
- Gradually increase the time outdoors each day, exposing them to more sunlight and wind.
By the end of the hardening off period, your seedlings will be ready to handle the outdoor environment.
Step 4: Transplanting outdoors
Once the danger of frost has passed and the soil is workable, it’s time to transplant your onion seedlings into the garden.
Choose the right location
Onions need a sunny spot with well draining soil. Prepare the soil by loosening it and mixing in compost or aged manure to provide nutrients.
Planting the seedlings
- Space your onion seedlings 4-6 inches apart, with rows about 12-18 inches apart.
- Plant the seedlings at the same depth they were growing indoors, being careful not to bury the bulb portion.
- Water thoroughly after planting to help the roots establish.
Step 5: Caring for onions in the garden
Watering
Keep the soil consistently moist but avoid overwatering, as onions dislike soggy conditions. Water deeply once or twice a week, depending on rainfall.
Fertilizing
As the onions grow, continue to fertilize every 2-3 weeks with a balanced fertilizer or one high in nitrogen to encourage leafy growth.
Weeding
Onions don’t compete well with weeds, so keep the area around them weed-free. Use mulch to suppress weeds and retain soil moisture.
Step 6: Harvesting Long day onions
Your onions are ready to harvest when their tops turn yellow and begin to fall over, typically in mid to late summer. To harvest:
- Gently lift the onions from the soil.
- Allow them to cure in a warm, dry place for 2-3 weeks.
- Once the outer skins are papery and the tops are fully dry, trim the leaves and store the onions in a cool, dark place.
Here’s a quick growing guide for your reference:
Growing long day onions from seed is a rewarding process that requires patience and care, but the payoff is worth it. By starting seeds indoors in late winter, providing the right conditions for growth, and preparing your seedlings for transplanting, you can enjoy a bountiful harvest of large, delicious onions by summer.
Let’s grow together! 💚
